Cannot cast from source type to destination type in C#

Questions & Answers
1

Hi, I’m trying to use the code this.

public static string CreateOptionTooltip(Item item)
	{
		if(item.getEquipable())
		{
			string tooltip = "";
			EquipmentItem equipItem = (EquipmentItem)item;
			for(int i = 0; i < equipItem.optionList.Count; i++)
			{
				ItemOption option = equipItem.optionList*;*

  •  		tooltip += option.itemOption.ToString();*

  •  		if(option.optionAmount >= 0)*

  •  			tooltip += "	+" + option.optionAmount.ToString();*

  •  		else*

  •  			tooltip += "	-" + option.optionAmount.ToString();*

  •  		tooltip += "

";*

  •  	}*

  •  	return tooltip;*

  •  }*

  •  else*

  •  {*

  •  	return "";*

  •  }*
  • }*
    The EquipmentItem Class is child class of Item like this.
    public class EquipmentItem : Item
    {
    public EquipmentItem()
    {
    //Here is initializing code.
    }
    }
    But when I call the method CreateOptionTooltip(Item item) in gaming
    there is error
    InvalidCast: Cannot cast from source type to destination type
    in line
    EquipmentItem equipItem = (EquipmentItem)item;
2

You can only cast up the inheritance tree, you can’t cast down the inheritance tree because not everything is guaranteed to exist.

Say you have a class:

class Foo {
    int index;
    float position;
}

And another class:

class Bar : Foo {
    void Do() {
    }
}

Now lets say you are given a variable of type Bar. This you can easily cast to a Foo because Bar inherits from Foo, ie. you’re casting up the tree. Once you have a Foo from your Bar, everything from Foo still exists in your variable, ie. you have access to index and position.

But what if you have a variable of type Foo. This you cannot cast to a Bar, simply because a Bar can Do(), while a Foo cannot. Therefore your Foo cannot be a Bar, but your Bar can be a Foo.

The same goes for the Finger-Thumb analogy. Think of Foo as a Finger and Bar as a Thumb. Your Thumb is a finger, ie. you can cast your Thumb to a Finger. But not all Fingers are Thumbs, you therefore cannot cast any Finger to a Thumb.

You therefore cannot cast an Item to a EquipmentItem, since not all items are equipment items, but you can cast any EquipmentItem to an Item, since all EquipmentItems are Items.